Vibratory screeds are frequently employed to impart vibrations to poured concrete for tamping and leveling of a concrete slab as it is finished. The screed eliminates the tedious manual labor involved in finishing the concrete, thereby increasing productivity in laying of the concrete. In addition, the screed improves the quality of the finished concrete slab by providing more accurately controlled slab surfaces, as well as controlling the crown.
U.S. Pat. No. 4,758,114 discloses a vibratory concrete screed, and as shown in that patent, a winch is mounted on each end of the screed and a cable is wound on the drum of each winch with the free end of each cable being attached to a stake or fixed object along the side of the concrete slab. By rotating each drum, the cable is wound on the drum to thereby move or traverse the screed along the concrete slab.
While it would be desirable to continuously move the screed along the concrete slab at a fixed rate of speed, in practice, the screed is normally moved in a start and stop manner. It is further desired that the speed of travel have "memory" so that when the movement is started it will be at the same speed as previously used.
In the past, screeds have been manually operated using an operator at each winch. More recently, power operated winches have been employed, in which the power for operation of the winches is taken off of the vibratory eccentric shaft of the screed. However, with a power take-off system, an extensive speed reduction mechanism is required, because the vibratory eccentric shaft is normally operating at a speed of about 2500 rpm, while the winch cable drum rotates at a speed of about 10 to 15 rpm.
With one type of power operated winch, power is taken off the eccentric shaft to drive a hydraulic pump, which in turn operates hydraulic motors that are connected to the winch cable drums. However, hydraulic systems of this type are expensive and have a relatively slow reaction in starting and stopping. There is also a potential of oil leaks from the hydraulic system onto the concrete surface which would destroy the surface characteristics of the concrete.
Other types of power driven winches have used expensive gear trains or variable speed transmissions in order to achieve the desired speed reduction to the winch cable drum.